When one uses a story/saying/information/anecdote that they recieved or overheard from another individual they know and they do not cite the source. In turn, implying that they themselves are the original source of information.

Maegan tells Leann that she talked to the band about their performance date change in Santa Cruz.
Later that evening, Leann tells a group of mutual friends that the performance date has been changed, without citing/acknowledging where she originally obtained this information. In turn, implying she spoke to the band herself, which could be construed to some as lying.
Thus, Leann commited social plagiarism.